The present invention is in the field of plastic films or sheets and more particularly relates to embossed and/or perforated thermoplastic films.
Embossed plastic films are used for a variety of purposes as a substitute for textiles, as in raincoats or other wearing apparel, diaper liners, shower curtains, hospital pads and drapes. Such film is also used in the packaging field, for example as bags and overwraps for articles such as clothing, etc., and for shopping bags.
Perforated films also have many useful applications. Films of this type are used in gardening and farming to prevent the growth of grass or weeds while permitting moisture to be transmitted through the film to the solid beneath. Perforated thermoplastic film is also used for making disposable diapers and in packaging of foods and other materials.
One generally common embossed film pattern is one which consists of a pattern of protruding bosses in the plastic film which extend up from only one surface of the film surface. These bosses are formed in the film such that the sides of the designs form channels which may extend diagonally or perpendicularly to the edges of the sheet.
An embossed thermoplastic film characterized in that one surface of the film is provided with a plurality of rows of protuberances which have the shape of pyramids with square bases which extend perpendicular to both the longitudinal and the transverse axes of the film is shown in U.S. Pat. No. 4,343,848. The protuberances are joined at the edges of the bases by flat valley portions which intersect each other at right angles. The embossed film has a low coefficient of friction and in creased embossed thickness.
An embossed film having latitudinally and longitudinally alternating hollow protuberances and depressions on both sides of the film is disclosed in U.S. Pat. No. 3,911,187.
Embossed film having a truncated pyramid pattern wherein the valleys separating the pyramids are continuous over the full length and width of the film is shown in U.S. Pat. No. 3,760,940. Examples of embossed film having a pattern of rounded top, rectangular base protuberances wherein the valleys extend perpendicular to the transverse and longitudinal axes of the film is shown in U.S. Pat. No. 3,484,835. Square pattern film produced by embossing on a traveling wire wherein the valleys extend parallel to the longitudinal and transverse axes of the film is shown in U.S. Pat. No. Re. 23,910. U.S. Pat. No. 3,137,746 discloses a film having a hexagonal, raised pattern on one surface thereof which is substantially oriented and slit between the embossments to produce a porous film. The hexagons are initially arranged so that continuous valleys exist on two opposite sides of the individual rows of protuberances. Embossing patterns as shown in these patents produce embossed films which have a high coefficient of friction. In some applications such as baby diapers, it is desirable that the coefficient of friction be kept low.
Embossed thermoplastic films such as polyethylene, polypropylene, polybutene-1, polyvinyl chloride, and other flexible thermoplastics normally extruded into film have been made by various methods. One method used to prepare embossed thermoplastic film is to extrude the thermoplastic material, e.g., polyethylene, from a conventional slot extrusion die onto a continuously moving, smooth, cooled casting surface, e.g., a chill roll. The engraved pattern may be applied to the chill roll and the film pressed to the roll while in the amorphous or molten stage by press rolls. Alternatively, the chill roll may be very smooth and the desired pattern in the film may be impressed into the film on the chill roll by means of an engraved or machined embossing roll which is pressed against the film and the chill roll to impress the pattern into the film as it is cooled on the chill roll. The softness of embossed film produced by chill casting is directly related to the density of the polyethylene resin used. In order to obtain different degrees of softness or stiffness, it is necessary to use a number of polyethylene resins having different densities. Thus, if it is desired to produce a relatively stiff embossed film, it is necessary to use more expensive polymers having high densities as the feed material to the slot die.
Embossed film rolls of poor conformation produce problems when running the film through fabricating machines or through a film printing apparatus.
An example of a method and apparatus for producing film according to the foregoing slot die-chill cast roll technique is shown in U.S. Pat. No. 3,374,303.
Another technique used for embossing plastic film has been the utilization of a heated engraved embossing roll in conjunction with a backup roll. The preformed strip of thermoplastic film normally at room temperature, is passed between the nip of a heated engraved roll and a backup roll and is embossed by being heated while in contact with the heated, engraved roller. The resultant embossed film usually has a very shallow and poorly defined pattern. An example of an apparatus and process for carrying out embossing of this type is shown in U.S. Pat. No. 3,176,058.
A third process for embossing thermoplastic film has been to pass the film over a heated roll or a series of heated rollers in order to heat the film to a softened state and then to contact the film with an embossing roller and to press the film against the embossing roller by a backup roller. Normally, the embossing roller and the backup rollers are cooled in order to freeze the embossed pattern into the film so that it may be immediately wound up into rolls, if desired. An apparatus and process for preparing an embossed film according to the foregoing is shown in U.S. Pat. No. 3,246,365.
A more recent process for embossing plastic material is shown in U.S. Pat. No. 3,950,480, wherein the film is heated by a nondirect contact heat source to raise the temperature of the film above its softening point and the film is then immediately fed between adjacent, counter-rotating embossing rollers, and thereby embossed.
One of the methods for perforating thermoplastic sheet or film is disclosed in U.S. Pat. No. 3,054,148, issued to Zimmerli, which is hereby incorporated by reference. The Zimmerli patent discloses a stationary drum having a molding element mounted around the outer surface of the drum which is adapted to rotate freely thereon. A vacuum chamber is employed beneath the screen or molding element to create a pressure differential between the respective surfaces of the thermoplastic sheet to cause the plasticized sheet to flow into the perforations provided in the molding element and thereby cause a series of holes to be formed in the sheet.
U.S. Pat. Nos. 4,155,693 and 4,157,237 illustrate types of screens or molding elements.
U.S. Pat. Nos. 4,252,516 and 4,317,792 disclose apparatus and method, respectively for manufacturing thermoplastic sheet having elliptical holes.
Sheets of flexible, reinforced laminated plastic material are finding increasing use in a wide variety of applications today. The advent of low cost thermoplastic material, e.g., polyethylene has increased the use of reinforced polyethylene as a covering for greenhouses, barns, temporary buildings, weatherproofing large buildings under construction, and as a covering material for materials stored in the open, e.g., lumber, grain, hay, and many other uses. Most reinforced thermoplastic sheet material is manufactured by laminating a web of strand material between two sheets of thermoplastic. Reinforced film is commonly made by heat bonding thermoplastic film through the openings provided in the web of reinforcing material, by applying adhesive to the web or to one or more of the innerfaces of the thermoplastic material and pressing the plastic material to either side of the reinforcing material, and by extruding one or more layers of thermoplastic material onto a reinforcing material to provide a unitary reinforced thermoplastic sheet. Among patents disclosing reinforced thermoplastic material and methods for their manufacture are U.S. Pat. Nos. 2,851,389; 3,214,320; 3,222,237, and 3,687,764.
The reinforcing material may be in the form of a prewoven scrim or woven mesh material having wide openings between the strands. The strands from which the web is prepared may be from either monofilament or multifilament materials. In some cases it is preferred to lay a uniformly arranged array of strands of reinforcing material into a web on one of the plastic sheets immediately prior to applying adhesive thereto or fusing the sheets together by heat. Machines for laying down a nonwoven scrim to provide reinforcement between sheets of plastic material are shown in U.S. Pat. Nos. 3,272,679; 3,156,027; and 1,914,801.
It is therefore a principal object of the present invention to provide a plastic film with geometric shapes built therein for controlling the coefficient of friction of the film.
It is an important object of the invention to provide a plastic film having geometric shapes constructed therein for controlling the coefficient of friction of the film which may be either perforated or non-perforated.
It is a particular object of the present invention to provide a plastic film with geometric shapes so constructed therein, that one surface area of the plastic film may be adjoined or fixed to another surface thereof by the mere application of pressure.
Other objects and advantages of the invention will be more readily apparent from the description and drawings herein- after.